Polymer dispersed liquid crystals (PDLCS) are a new type of light shutter which has been studied intensively in recent years. In a PDLC light shutter, the dispersed liquid crystal exists in the form of droplets with a diameter of about 1 micron. The configuration of the liquid crystal inside the droplet depends on the size and shape of the droplet as well as on the boundary condition and the external field. From the application point of view, PDLCs have unique optical properties: opaque in a field-OFF condition and transparent in a field-ON condition. They do not require polarizers and have very high transmittance in the ON-state. PDLCs may be used for switchable windows, direct view and projection displays.
Starting from a uniform mixture of liquid crystal and polymer, phase separation can be induced by temperature change (Temperature Induced Phase Separation or TIPS), solvent evaporation (Solvent Induced Phase Separation or SIPS), and polymerization of the polymer precursor in the mixture (Polymerization Induced Phase Separation or PIPS).
The size of the droplets can be controlled by the phase separation conditions. If the droplet size is small (about 1 micron), the polymer dispersed liquid crystal strongly scatters light in the field-off state and is transparent in the field-on state. With a bigger droplet size, the light scattering becomes much weaker. If the liquid crystal is nematic, the PDLC cell looks transparent. When nematic liquid crystals are replaced with cholesteric liquid crystals of pitch comparable to the wavelength of visible light, the display exhibits the selective reflection property of cholesteric liquid crystals.